V - Effets de la température et de l’environnement sur la propagation des fissures de fatigue dans un alliage de Ti

Effects of temperature and environment interactions on fatigue crack propagation in a Ti alloy

Abstract

Titanium alloys are well known as candidates for aeronautical applications in turbine engines because they remain efficient at moderately elevated temperatures. The permanent need to increase the operating temperature requires intensive testing on the fatigue crack propagation behaviour of new generation alloys. To analyse the effect of environment, fatigue crack growth (FCG) experiments were carried out on a TA6V alloy in several atmospheres at 300°C with different total pressures and water vapour partial pressures. Water vapour embrittlement has been identified as the governing mechanism for near-threshold propagation at 300°C. The Ti6246 alloy dedicated to higher operating temperatures has been studied at temperatures ranging from 20°C to 500°C. At low rates the effect of temperature is the most pronounced, leading to an increase of crack growth with increasing temperature. A critical temperature in the range 465°C to 500°C above which a time-dependent damage mechanism is operative at mid-rates with a substantial decrease in FCG resistance is detected when frequency decreases from 35 Hz. to 0.1 Hz. Dwell loadings performed at 500°C give growth rates comparable to that of the sinusoidal loading at 0.5 Hz and 0.1 Hz supporting that a corrosion fatigue mechanism type of damage seems to be operative at 500°C.